Balls having a two-piece construction are generally most popular with the average recreational golfer because they provide a very durable ball while also providing maximum distance. Two-piece balls are made with a single-solid core, usually comprised of a crosslinked rubber, which is encased by a cover material. Typically the solid core is made of a polybutadiene blend which is chemically crosslinked through the use of a combination of initiators and crosslinking agents dispersed throughout the rubber stock. Common additives to the rubber stock are zinc diacrylate (ZDA) to affect the optimal cross linking level as well as various peroxide initiators that will affect the time rate of crosslinking. The core material is typically compression molded and the cover is applied in a separate operation, either compression molded or injection molded around the core. The cover material is often a tough, cut-resistant elastomer.
Regardless of the form of the ball, players generally seek a golf ball construction that has particular play characteristics of velocity and spin, which match their swing style and club preference. It is well know in the golf ball industry that both initial ball velocity and spin have both been determined to be substantially dependent on the compression of the core. Typically the optimal level of crosslinking of the core material is predetermined and adjusted by the level of crosslinking agent to arrive at the desired core compression. Once players select a ball construction, they want consistent play characteristics between individual balls. Thus, to maintain consistent play performance between individual balls of a particular construction, manufacturers must produce golf balls with very consistent level of crosslinking of the core material.
The golf ball core is formed within a compression type mold, which is heated to accelerate the core stock crosslinking reaction. The magnitude of temperature of the mold will affect the time rate of crosslinking of the core stock and therefore will affect the length of time the stock must reside in the mold (or cure time) to achieve complete or optimal level of crosslinking. The mold is typically comprised of multiple cavities to facilitate high volume manufacturing. In different locations within the core mold, cavity temperature will vary, for example cavity locations along the outside perimeter will be cooler than the cavity locations in the inner regions of the mold. Therefore, in order to achieve complete crosslinking for all cores in all cavity locations, the cure time must be adjusted for the coolest regions that will have the lowest rate of reaction of the core stock.
When compression molding cores, the molding cycle times are generally a compromise between productivity levels and the consistency of golf ball core compressions. Shorter cure cycles increase the productivity levels of golf ball core molding equipment by facilitating production of more golf ball cores per molding machine. With shorter cure cycles, however, the golf ball cores in cooler regions of the mold are typically only cured to between 85% and 95% of optimal cure. This less than 100% cure state of the golf ball core makes it difficult to maintain a tight range of core compressions or consistent compressions. Also, because of the shorter cure cycle, the amount of crosslinking agent is often increased to reach the desired golf ball compression. Standard compression molding occurs at a cavity pressure of 4000 p.s.i., at a temperature of 335° F. for a time of 11 minutes.
As the cure state of the core production population reaches 100%, the crosslinking agent can be maintained at a comparatively lower level, saving substantial amounts of the chemical. In turn, making the cores less expensive. Also, a core population with cure state near 100% negates the inconsistencies in core compressions caused by variations in the molding temperature.
One method of forming a golf ball core is disclosed in U.S. patent application Ser. No. 09/145,882, filed Sep. 2, 1998 by D. Ladd et al. This is a method for forming a golf ball core with at least one mantle layer formed around a center. First, a solid spherical center is prepared by one of the conventional compression, injection molding, or winding techniques. Then, a mixture is formed into a plurality of shells having the desired shape of the mantle layer, and the shells are heat cured until they are sufficiently rigid to maintain their desired shape. Finally, the shells are assembled around the golf ball center to form the core, and the core is subjected to a second curing cycle, such as compression molding, to further crosslink the mixture and form a cured golf ball core.
Thus, golf ball manufacturers are continually searching for new ways in which to make golf balls that perform well, that maintain the productivity of golf ball core molding equipment, and at the same time that provide a cure state of the core near 100%. The present invention provides such a method.